In the technology of measurements and automation for processes, the measurement of physical parameters of a fluid flowing in a pipeline, parameters such as e.g. mass flow rate, density and/or viscosity, measuring devices are often used that effect reaction forces in the fluid, such as e.g. Coriolis forces corresponding to the mass flow rate, inertial forces corresponding to the density or frictional forces corresponding to the viscosity, etc., by means of a vibratory measurement pickup, placed in the course of the fluid-guiding pipeline and flowed-through, or traversed, by the fluid during operation, together with a measuring and operating circuit connected thereto. The measuring devices produce, derived from these forces, a measurement signal representing the particular mass flow rate, the particular viscosity and/or the particular density of the fluid. Vibratory measurement pickups are described e.g. in WO-A 03/076880, WO-A 02/37063, WO-A 01/33174, WO-A 00/57141, WO-A 99/39164, WO-A 98/07009, WO-A 95/16897, WO-A 88/03261, U.S. 2003/0208325, U.S. Pat. No. 6,513,393, U.S. Pat. No. 6,505,519, U.S. Pat. No. 6,006,609, U.S. Pat. No. 5,869,770, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,602,346, U.S. Pat. No. 5,301,557, U.S. Pat. No. 5,218,873, U.S. Pat. No. 5,069,074, U.S. Pat. No. 4,876,898, U.S. Pat. No. 4,733,569, U.S. Pat. No. 4,660,421, U.S. Pat. No. 4,524,610, U.S. Pat. No. 4,491,025, U.S. Pat. No. 4,187,721, EP-A 553 939, EP-A 1 001 254 or EP-A 1 281 938.
For guiding the fluid, the measurement pickups include at least one measuring tube held in an e.g. tubular or box-shaped support frame. The measuring tube has a curved or straight tube segment, which is caused to vibrate during operating, driven by an electromechanical exciter arrangement, in order to produce the above-mentioned reaction forces. For registering, especially inlet-end and outlet-end, vibrations of the tube segment, the measurement pickups additionally have an electrophysical sensor arrangement reacting to movements of the tube segment. In the case of Coriolis mass flow meters for a medium flowing in a pipeline, the measurement of mass flow rate rests, for example, on allowing the medium to flow through the measuring tube inserted into the pipeline and vibrating the measuring tube during operation, whereby the medium experiences Coriolis forces. These, in turn, effect that inlet-end and outlet-end regions of the measuring tube oscillate shifted in phase relative to one another. The size of these phase-shifts serves as a measure for the mass flow rate. The oscillations of the measuring tube are, therefore, registered by means of two oscillation sensors of the aforementioned sensor arrangement separated from one another along the length of the measuring tube and converted into oscillation measurement signals, from whose phase shift with respect to one another the mass flow rate is derived.
Already the above-referenced U.S. Pat. No. 4,187,721 mentions that usually the instantaneous density of the flowing medium is also measurable with Coriolis mass flow meters, and, indeed, on the basis of a frequency of at least one of the oscillation measurement signals delivered by the sensor arrangement. Moreover, usually also a temperature of the fluid to be measured is directly measured in suitable manner, for example by means of a temperature sensor arranged on the measuring tube. It can thus be assumed without more—even when not expressly stated—that, in any case, also density and temperature of the medium are measured by means of modern Coriolis mass flow measuring devices, especially since these are always needed in the case of mass flow rate measurement anyway for the compensation of measurement errors stemming from fluctuating fluid density; see, in this connection, especially the already mentioned WO-A 02/37063, WO-A 99/39164, U.S. Pat. No. 5,602,346 or also WO-A 00/36379.
In the use of measurement pickups of the described kind, it has, however, been found, that, in the case of inhomogeneous media, especially two or more phase fluids, the oscillation measurement signals derived from the oscillations of the measuring tube, and especially also the mentioned phase shift, are subject to considerable fluctuations, despite keeping the viscosity and density of the individual fluid phases practically constant and/or appropriately taking them into consideration, such that these signals and phase shift can, in some cases, become completely unusable without remedial measures. Such inhomogeneous media can be, for example, liquids, into which, as can be practically unavoidable in the case of dosing or bottling processes, a gas, especially air, present in the pipeline, is entrained or out of which a dissolved fluid, e.g. carbon dioxide, outgasses and leads to foam formation. Wet, or saturated, steam is another example of such inhomogeneous media which can be named.
Already in U.S. Pat. No. 4,524,610, a possible cause of this problem is indicated for the operation of vibratory measurement pickups, namely the circumstance that inhomogeneities entrained into the measuring tube by the fluid, inhomogeneities such as e.g. gas bubbles, deposit on its inner wall and so can influence the oscillatory behavior to a considerable degree. For avoiding this problem, it is additionally proposed to so install the measurement pickup that the straight measuring tube runs essentially vertically, in order to prevent the depositing of such disturbing, especially gaseous, inhomogeneities. This is, however, a very special solution, which is only realizable in very limited circumstances, especially in the technology of measurements in industrial processes. On the one hand, in this case the pipeline, into which the measurement pickup is to be installed, might have to be fitted to the pickup, instead of the reverse, which is something which a user can prove to be not too interested in hearing. On the other hand, it is possible, as already mentioned, that the measuring tubes can be curved, so that the problem can then not be solved anyway by an adapting of the orientation of installation. It has, moreover, become evident that the mentioned corruption of the measurement signal is not really significantly avoided anyway by the use of a vertically installed, straight measuring tube. Moreover, further attempts to avoid, in this way, the fluctuations of the thus-produced measurement signal in the case of flowing fluid have likewise proved unsuccessful.
Similar causes, as well as their effects on the measurement accuracy, in the case of determining mass flow rate have been discussed, for example, also in JP-A 10-281846, WO-A 03/076880 or U.S. Pat. No. 6,505,519. While, for decreasing the measurement errors associated with two, or more, phase fluids, WO-A 03/076880 proposes a flow-, respectively fluid-, conditioning preceding the actual flow measurement, both JP-A 10-281846 and U.S. Pat. No. 6,505,519 each prefer a correction of the flow measurement, especially the mass flow rate measurement, resting on the oscillation measurement signals. This correction utilizes, for example, pre-trained, possible even adaptive, classifiers for the oscillation measurement signals. The classifiers can, for example, be designed as Kohonen maps or neural networks, and conduct the correction either on the basis of some few parameters measured in operation, especially the mass flow rate and the density, as well as further features derived therefrom, or also with use of an interval of the oscillation measurement signals encompassing one or more oscillation periods.
The use of such classifiers includes, for example, the advantage that, in comparison to conventional Coriolis mass flow rate/density meters, little or no changes need to be made on the measurement pickup, be it regarding the mechanical structure, the exciter arrangement, or the operating circuit driving such, which are especially matched to the particular application.
However, a significant disadvantage of such classifiers is, among other things, that, as compared to conventional Coriolis mass flow meters, considerable changes are required in the area of producing the measured value, above all with regard to the analog-to-digital converter being used and with regard to the microprocessors. As, in fact, also described in the U.S. Pat. No. 6,505,519, such a signal evaluation requires, for example in the digitizing of the oscillation measurement signals, which can have an oscillation frequency of about 80 Hz, a sampling rate of about 55 kHz, or more, in order to achieve a sufficient accuracy. Expressed differently, the oscillation measurement signals have to be sampled using a sampling ratio of far above 600:1. Beyond this, the firmware stored and executed in the digital measurement circuit becomes correspondingly complex.
A further disadvantage of such classifiers is also to be seen in the fact that they have to be trained and correspondingly validated for the conditions of measurement actually existing during operation of the measurement pickup, be it the particular details of the installation, the fluid to be measured and its usually varying properties, or other factors influencing the measurement accuracy. Because of the high complexity of the interaction of all of these factors, the training and its validation can usually only occur on site and individually for every measurement pickup, a feature which, in turn, causes considerable complications to be associated with the startup of the measurement pickup. Furthermore, it has been found that such classifier algorithms, on the one hand because of the high complexity, on the other hand because of the fact that usually an appropriate, physical-mathematical model with technically relevant or comprehensible parameters is not explicitly present, classifiers exhibit a very low transparency and are thus often difficult to communicate. Associated with this, of course, considerable reservations can arise on the part of the customer, with such acceptance problems on the part of the customer especially arising, when the classifier being used is self-adapting, for example a neural network.